1. Field of the Invention
The present invention relates to processes for reducing the level of sulfur in a sulfur-containing gas. In particular, the invention relates to the use of improved sulfur dioxide-sorbent calcium alkali slurries, which include a calcium-reactive alumina or silica source, in the desulfurization of sulfur-containing flue gases, and methods for improving the sulfur dioxide absorbing capabilities of such slurries.
2. Description of the Related Art
Coal represents one of the most bountiful sources of energy in the world today. For example, it has been estimated that the known coal reserves in the U.S. alone could supply sufficient energy for domestic consumption for several hundred years. Unfortunately much of this coal contains high levels of sulfur which, when the coal is burned, is released into the atmosphere, generally in the form of sulfur dioxide. One of the most serious environmental problems associated with such sulfur emissions is the generation of atmospheric sulfuric acid, resulting in so-called "acid rain."
Attempts at controlling sulfur dioxide emissions from coal burning plants have led to the development of a number of advanced systems and processes for flue gas desulfurization. Fluidized-bed combustion, lime injection, and flue gas desulfurization are some of the examples. In these processes, limestone has been used as a sorbent which forms primarily calcium sulfate at a temperature above 700.degree. C. Regeneration of the sorbent has been a difficult problem because of the high chemical stability of the sulfate. Yet, regeneration is desirable from the points of view of conservation, cost, and ecology. As a result, a considerable amount of research effort has been expended in developing alternate sorbents which are regenerative as well as reactive to sulfur dioxide.
Fluidized bed combustion (FBC) and scrubbers for flue gas desulurization (FGD) represent two of the more promising advanced processes for power generation. FBC relates to the combustion of coal with limestone particles as the bed material, and has received increasing attention as a promising and versatile technology for clean power generation. Equally promising has been FGD, wherein sulfur-reactive sorbents are employed to remove slfur from flue gases prior to their venting into the atmosphere. In developing the technologies for FBC and FGD, a search for sorbents more effective than limestone, especially ones which are economically regenerative, has been a challenging task.
Flue gas desulfurization by the means of spray dryer absorber and bag filter or electrostatic precipitator has recently received much attention. In the spray dryer/bag filter system, flue gas is contacted with a fine spray of an aqueous solution or slurry of a reactive alkali (typically lime), with SO.sub.2 removal and drying occuring simultaneously. The sulfur dioxide is absorbed into the water droplet during the constant rate period of drying until it shrinks to the extent that the particles touch each other. During the following falling rate period, the remaining water diffuses through the pores of agglomerated particles until the solids establish pseudo-equilibrium with the humid environment of spray dryer.
The third stage of drying may be called the second-falling rate period. Any drying/mass transfer during this period is limited by the diffusion of moisture from within tightly packed particles. The first two stages take place exclusively in the spray dryer. The majority of pseudo-equilibrium period occurs in the duct joining spray dryer and bag filter and in the bag filter itself. Since not all moisture is removed from the solids in the spray dryer, the remaining moisture promotes further removal of SO.sub.2 in the bag filter. Therefore the total SO.sub.2 removal in the system is a sum of removal in the spray dryer and bag filter.
The recycle of product solids is among the options that have been tested to increase the utilization of reagent. Numerous reports indicate that recycle of product solids and fly ash results in substantial improvement of reagen utilization and SO.sub.2 removal. This option provides a higher Ca(OH).sub.2 concentration in the slurry feed at the same Ca(OH).sub.2 stoichiometry (moles of Ca(OH).sub.2 fed to the system/moles of SO.sub.2 in the feed gas). In one pilot plant, increasing the recycle ration (g solids recycled/g fresh Ca(OH).sub.2) from 6:1 to 12:1 increased SO.sub.2 removal in the spray dryer from 70% to 80% at stoichiometry 1.0 (Blythe et al., 1983, Proceedings: Symposium or Flue Gas Desulfurization, Vol. 2, NTIS PB84-110576). In another installation, compared to once-thru tests, recycle tests gave 10 to 15% more SO.sub.2 removal at stoichiometry 1.5 (Jankura et al., presented at the Eighth EPA/EPRI . Symposium on Flue Gas Desulfurization, New Orleans, La., 1983).
Another option enhancing lime utilization uses the recycle of both solids captured downstream in the spray dryer and solids from the baghouse. However, removal does not appear to be significantly different when either spray dryer solids or fabric filter solids are employed as the recycled material. At stoichiometry 1.0 the removal increased from 53% when no recycle was employed to 62% with 0.5:1 recycle ration. When ash content in the feed slurry increased from 5% to 20%, SO.sub.2 removal in the spray dryer increased from 80% to 92% for stoichiometry 1.6 (Jankura et al., 1983).
U.S. Pat. No. 4,279,873, to Felsvang et al., relates several experiments investigating the effects of fly ash recycle and proved it to be beneficial for SO.sub.2 removal in a spray dryer. It was found that substantially higher removal of SO.sub.2 may be achieved when recycling the fly ash and Ca(OH).sub.2 than when recycling Ca(OH).sub.2 alone. Corresponding efficiencies for stoichiometry 1.4, 500 ppm inlet SO.sub.2, and comparable solids concentration were 84% and 76%, respectively. For the same stoichiometry and SO.sub.2 concentration, removal was only 67% for the simple once-thru process. At low SO.sub.2 concentration and high recycle rations, over 90% removal was achieved even at extremely low stoichiometries. At 548 ppm SO.sub.2, 25:1 recycle, 0.76 stoichiometry and at 170 ppm SO.sub.2, 110:1 recycle, 0.39 stoichiometry, SO.sub.2 removal was 93.8% and 97.8%, respectively.
Removal efficiencies up to 65% were reported with a slurry of highly alkaline (20% CaO) fly ash only (Hurst and Bielawski, Proceedigs: Symposium on FGD, EPA-600/9-81-019b, 853-860, 1980). In another experiment, 25% SO.sub.2 removal was achieved when spraying slurried fly ash collected from a boiler burning 3.1% sulfur coal (Yeh et al., Proceedings: Symposium on Flue Gas Desulfurization, EPRI CS-2897, 821-840, 1983). A weak trend was found in a study of 22 samples of fly ashes that a slurry with a higher total slurry alkalinity tended to have a higher SO.sub.2 capture (Reed et al., Environ. Sci. Technol., 18, 548-552, 1984).
Therefore, while it is clear that desulfurization processes employing flue gas scrubbers represents an important advance, it is equally clear that such techniques presently have economic and technical drawbacks, not the least of which is the low degree of reagent utilization. While recycle of product solids with fly ash has resulted in some improvement, such processes are still not economically feasible for certain applications, and much room remains for the improvement of reagent utilization in such systems.